Condensation polymers, primarily polyesters and polyamides, are produced in very large volumes. There is considerable incentive to increase the utilization of already existing production equipment and to provide new equipment incorporating higher efficiency processes. One method long known for improving utilization is to increase the conversion rate to high molecular weight finished product from lower molecular weight prepolymer in the so-called finishing step typical of many commercial condensation polymer production facilities.
The polycondensation reactions which result in the formation of high molecular weight polymer are equilibrium reactions. Thus, in order to drive the reaction to higher molecular weight, it is desirable to quickly remove the volatile reaction products which participate in the reverse (depolymerization) reactions. It has long been known in the art to accomplish this removal by the application of vacuum, see, for example, the Encyclopedia of Chemical Processing and Design, J. McKetta, ed., vol. 40, P. 156, Marcel Dekker, Inc., New York, 1992, or in the alternative, by the application of an inert gas purge to the finisher, see, for example, the Encyclopedia of Polymer Science and Engineering, 2nd ed., vol 12, p. 30, John Wiley & Sons, New York, 1990.
Further known in the art, and taught in the references hereinabove cited, is the desirability to maximize surface to volume ratio of the polymer during the finishing process, which is preferably accomplished in a melt-film forming apparatus such as is described in the parent case hereof. Since in many commercial scale operations, the rate of reaction is known to be mass-transfer limited, continuous renewal of the melt surface by melt agitation or mixing is of great importance.
Bhatia, U.S. Pat. No. 5,434,239, discloses a process for production of high molecular weight polyester in which application of an inert gas sweep with a high gas velocity at atmospheric pressure to an agitated melt in a melt-film forming apparatus enhances conversion rate of polyesters to high molecular weight over lower gas velocities or vacuum.
Sasaki et al. (U.S. Pat. No. 4,612,363) discloses a process whereby high volumetric flow rates of inert gas applied to a very thin non-agitated melt film induces very high rates of polymerization in the production of high molecular weight polyesters.
However, it has long been recognized in the art (Kemnitz et al., GD R46 692) that the application of large volumes of gas to an agitated melt is limited and complicated by excessive disruption of and entrainment of the melt, with resulting problems of process control and product quality. Thus, for example, the process of Sasaki et al. is limited to non-agitated and very thin films, and would not be suitable for use in a melt film finishing device providing an agitated melt.
Hussein (GB 2,118,565A) discloses a solid phase polymerization process utilizing a purge of air or inert gas at atmospheric or reduced pressure to achieve modest conversion rates of polyesters. No particular combination is said to be preferred; indeed Hussein suggests that all configurations are equivalent in effectiveness.